Function and expression of melatonin receptors on human pancreatic islets

Diabetes-associated Genetic Variation in MTNR1B and Its Effect on Islet Function

Context

Multiple common genetic variants have been associated with type 2 diabetes, but the mechanism by which they predispose to diabetes is incompletely understood. One such example is variation in MTNR1B, which implicates melatonin and its receptor in the pathogenesis of type 2 diabetes.

Objective

To characterize the effect of diabetes-associated genetic variation at rs10830963 in the MTNR1B locus on islet function in people without type 2 diabetes.

Design

The association of genetic variation at rs10830963 with glucose, insulin, C-peptide, glucagon, and indices of insulin secretion and action were tested in a cohort of 294 individuals who had previously undergone an oral glucose tolerance test (OGTT). Insulin sensitivity, β-cell responsivity to glucose, and Disposition Indices were measured using the oral minimal model.

Setting

The Clinical Research and Translation Unit at Mayo Clinic, Rochester, MN.

Participants

Two cohorts were utilized for this analysis: 1 cohort was recruited on the basis of prior participation in a population-based study in Olmsted County. The other cohort was recruited on the basis of TCF7L2 genotype at rs7903146 from the Mayo Biobank.

Intervention

Two-hour, 7-sample OGTT.

Main Outcome Measures

Fasting, nadir, and integrated glucagon concentrations.

Results

One or 2 copies of the G-allele at rs10830963 were associated with increased postchallenge glucose and glucagon concentrations compared to subjects with the CC genotype.

Conclusion

The effects of rs10830963 on glucose homeostasis and predisposition to type 2 diabetes are likely to be partially mediated through changes in α-cell function.

Monoamines' role in islet cell function and type 2 diabetes risk

The two monoamines serotonin and melatonin have recently been highlighted as potent regulators of islet hormone secretion and overall glucose homeostasis in the body. In fact, dysregulated signaling of both amines are implicated in β-cell dysfunction and development of type 2 diabetes mellitus (T2DM). Serotonin is a key player in β-cell physiology and plays a role in expansion of β-cell mass. Melatonin regulates circadian rhythm and nutrient metabolism and reduces insulin release in human and rodent islets in vitro. Herein, we focus on the role of serotonin and melatonin in islet physiology and the pathophysiology of T2DM. This includes effects on hormone secretion, receptor expression, genetic variants influencing β-cell function, melatonin treatment, and compounds that alter serotonin availability and signaling.

The Role of the Circadian Rhythm in Dyslipidaemia and Vascular Inflammation Leading to Atherosclerosis

Cardiovascular diseases (CVD) are among the leading causes of death worldwide. Many lines of evidence suggest that the disturbances in circadian rhythm are responsible for the development of CVDs; however, circadian misalignment is not yet a treatable trait in clinical practice. The circadian rhythm is controlled by the central clock located in the suprachiasmatic nucleus and clock genes (molecular clock) located in all cells. Dyslipidaemia and vascular inflammation are two hallmarks of atherosclerosis and numerous experimental studies conclude that they are under direct influence by both central and molecular clocks. This review will summarise the results of experimental studies on lipid metabolism, vascular inflammation and circadian rhythm, and translate them into the pathophysiology of atherosclerosis and cardiovascular disease. We discuss the effect of time-respected administration of medications in cardiovascular medicine. We review the evidence on the effect of bright light and melatonin on cardiovascular health, lipid metabolism and vascular inflammation. Finally, we suggest an agenda for future research and recommend on clinical practice.

Agomelatine inhibits platelet aggregation through melatonin receptor-dependent and independent mechanisms

AIMS

Melatonin is known to inhibit platelet aggregation induced by arachidonic acid (AA). In the present study we investigated whether agomelatine (Ago), an antidepressant with agonist activity at melatonin receptor 1 (MT1) and MT2 could reduce platelets aggregation and adhesion.

MAIN METHODS

Human platelets from healthy donors were used to test the in vitro effects of Ago in the presence of different platelet activators. We performed aggregation and adhesion assays, thromboxane B₂ (TxB₂), cAMP and cGMP measurements, intra-platelet calcium registration and flow cytometry assays.

KEY FINDINGS

Our data revealed that different concentrations of Ago reduced AA- and collagen-induced human platelet aggregation in vitro. Ago also reduced AA-induced increase in thromboxane B₂ (TxB₂) production, intracellular calcium levels and P-selectin expression at plasma membrane. The effects of Ago in AA-activated platelets were likely dependent on MT1 as they were blocked by luzindole (a MT1/MT2 antagonist) and mimicked by the MT1 agonist UCM871 in a luzindole-sensitive manner. The MT2 agonist UCM924 was also able to inhibit platelet aggregation, but this response was not affected by luzindole. On the other hand, although UCM871 and UCM924 reduced collagen-induced platelet aggregation and adhesion, inhibition of collagen-induced platelet aggregation by Ago was not mediated by melatonin receptors because it was not affected by luzindole.

SIGNIFICANCE

The present data show that Ago suppresses human platelet aggregation and suggest that this antidepressant may have the potential to prevent atherothrombotic ischemic events by reducing thrombus formation and vessel occlusion.

Sleep Patterns, Eating Behavior and the Risk of Noncommunicable Diseases

Sleep is extremely important for the homeostasis of the organism. In recent years, various studies have been carried out to address factors related to sleep patterns and their influence on food choices, as well as on the onset of chronic noncommunicable diseases. The aim of this article is to provide a scientific literature review on the possible role of sleep patterns on eating behavior and the risk of noncommunicable diseases. A search was performed on Medline (PubMed interface) using several keywords (e.g., "Factors Influencing Sleep" OR "Sleep and Chronic Diseases"). Articles published between 2000 and the present date that relate sleep to cyclic metabolic processes and changes in eating behavior were selected. Changes in sleep patterns are increasingly detected today, and these modifications are mainly caused by work and lifestyle conditions as well as a growing dependence on electronic devices. Sleep deprivation and the resultant short sleep duration lead to an increased appetite via an increase in the hunger hormone (ghrelin) and a decrease in the satiety hormone (leptin). Nowadays, sleep is undervalued, and thus often impaired, with consequences for the performance of various body systems. Sleep deprivation alters physiological homeostasis and influences eating behavior as well as the onset of chronic diseases.

Association of rotating night shift work, CLOCK, MTNR1A, MTNR1B genes polymorphisms and their interactions with type 2 diabetes among steelworkers: a case-control study

BACKGROUND

The purpose of this study is to investigate the association of rotating night shift work, CLOCK, MTNR1A, MTNR1B genes polymorphisms and their interactions with type 2 diabetes among steelworkers.

METHODS

A case-control study was conducted in the Tangsteel company in Tangshan, China. The sample sizes of the case group and control group were 251 and 451, respectively. The logistic regression, log-linear model and generalized multifactor dimensionality (GMDR) method were used to investigate the interaction between circadian clock gene, melatonin receptor genes and rotating night shift work on type 2 diabetes among steelworkers. Relative excess risk due to interaction (RERI) and attributable proportions (AP) were used to evaluate additive interactions.

RESULTS

Rotating night shift work, current shift status, duration of night shifts, and average frequency of night shifts were associated with an increased risk of type 2 diabetes after adjustment for confounders. Rs1387153 variants in MTNR1B was found to be associated with an increased risk of type 2 diabetes, which was not found between MTNR1A gene rs2119882 locus, CLOCK gene rs1801260 locus and the risk of type 2 diabetes. The association between rotating night shift work and risk of type 2 diabetes appeared to be modified by MTNR1B gene rs1387153 locus (RERI = 0.98, (95% CI, 0.40-1.55); AP = 0.60, (95% CI, 0.07-1.12)). The interaction between MTNR1A gene rs2119882 locus and CLOCK gene rs1801260 locus was associated with the risk of type 2 diabetes (RERI = 1.07, (95% CI, 0.23-1.91); AP = 0.77, (95% CI, 0.36-1.17)). The complex interaction of the MTNR1A-MTNR1B-CLOCK-rotating night shift work model based on the GMDR methods may increase the risk of type 2 diabetes (P = 0.011).

CONCLUSIONS

Rotating night shift work and rs1387153 variants in MTNR1B were associated with an increased risk of type 2 diabetes among steelworkers. The complex interaction of MTNR1A-MTNR1B-CLOCK-rotating night shift work may increase the risk of type 2 diabetes.

The Association between Dietary Iron Intake, SNP of the MTNR1B rs10830963, and Glucose Metabolism in Chinese Population

Type 2 diabetes is associated with both dietary iron intake and single-nucleotide polymorphism (SNP) of intronic rs10830963 in melatonin receptor 1B (MTNR1B); however, it is unclear whether they interact. The aim of this study was to examine the associations between dietary iron intake, SNP of rs10830963, and glucose metabolism. Data were obtained from the Shanghai Diet and Health Survey (SDHS) during 2012-2018. Standardized questionnaires were carried out through face-to-face interviews. A 3-day 24 h dietary recall was used to evaluate dietary iron intake. Anthropometric and laboratory measurements were applied. Logistic regression and general line models were used to evaluate the association between dietary iron intake, SNP of the MTNR1B rs10830963, and glucose metabolism. In total, 2951 participants were included in this study. After adjusting for age, sex, region, years of education, physical activity level, intentional physical exercise, smoking status, alcohol use, and total energy, among G allele carriers, dietary iron intake was associated with a risk of elevated fasting glucose, higher fasting glucose, and higher HbA1c, while no significant results were observed among G allele non-carriers. The G allele of intronic rs10830963 in MTNR1B potentially exacerbated unfavorable glucose metabolism with the increasing dietary iron intake, and it was possibly a risk for glucose metabolism homeostasis in the Chinese population.

The effect of melatonin on glycoprotein levels and oxidative liver injury in experimental diabetes

In this present study, the duration of melatonin (Mel) administered to diabetic rats was prolonged so as to examine its effects on the biochemical liver parameters of diabetic rats. In the experiment, Male Sprague Dawley rats were divided randomly into five groups; the control, diabetic + Mel, diabetic, diabetic + insulin, and diabetic + Mel + insulin. Diabetes mellitus was induced by administration of a single dose of streptozotocin (60 mg/kg) intraperitoneally and rats were given vehicle as a solvent for Mel every day for 12 weeks. In the diabetic + Mel group, diabetic rats were administered Mel (10 mg/kg/day) for 12 weeks to treat diabetes. The diabetic + insulin group were diabetic rats given insulin (6 U/kg) subcutaneously for 12 weeks. The diabetic + Mel + insulin rats received insulin and Mel at the same dose and time. At the end of the experiment, the animals were decapitated and liver tissues were taken. The protective effect of Mel on liver tissue of diabetic rats was investigated, total antioxidant status, total oxidant status, reactive oxygen species, oxidative stress index, adenosine deaminase, xanthine oxidase, paraoxonase 1, sodium/potassium ATPase, myeloperoxidase, γ-glutamyl transferase, sorbitol dehydrogenase, tumor necrosis factor-alpha, homocysteine, nitric oxide, glucose-6-phosphate dehydrogenase, and glycoprotein levels were determined in liver tissues. Treatment with Mel and/or insulin has been found to have a protective effect on biochemical parameters. The results showed that administration of Mel to diabetic rats prevented the distortion of the studied biochemical parameters of liver tissues.

A novel combination of sitagliptin and melatonin ameliorates T2D manifestations: studies on experimental diabetic models

INTRODUCTION

Type 2 diabetes (T2D) is an endocrine disorder characterized by hyperglycemia, insulin resistance, dysregulated glucose and lipid metabolism, reduced pancreatic β-cell function and mass, and a reduced incretin effect. Circadian rhythm disruption is associated with increased T2D risk. We have investigated the therapeutic potential of a combination of melatonin (M) and sitagliptin (S), a dipeptidyl peptidase IV (DPP-IV) inhibitor, in the amelioration of T2D manifestations in high-fat diet (HFD) induced T2D mouse model and also on β-cell proliferation under gluco-lipotoxicity stress in vitro.

METHODS

For in vivo study, mice were fed with HFD for 25 weeks to induce T2D and were treated with monotherapies and S + M for four weeks. For the in vitro study, primary mouse islets were exposed to normal glucose and high glucose + palmitate to induce gluco-lipotoxic stress.

RESULTS

Our results suggest that monotherapies and S + M improve metabolic parameters and glyco-lipid metabolism in the liver and adipose tissue, respectively, and improve mitochondrial function in the skeletal muscle. Moreover, it increases peripheral insulin sensitivity. Our in vitro and in vivo studies suggest that β-cell mass was preserved in all the drug-treated groups.

CONCLUSION

The combination treatment is superior to monotherapies in the management of T2D.

The neuroprotective effects of melatonin against diabetic neuropathy: A systematic review of non-clinical studies

Backgrounds: Diabetes can cause diabetic neuropathy (DN), a nerve injury. High blood sugar (glucose) levels can harm nerves all over your body. The nerves in your legs and feet are the most commonly affected by DN. The purpose of this study was to conduct a review of melatonin’s potential neuroprotective properties against DN.

Method: A full systematic search was conducted in several electronic databases (Scopus, PubMed, and Web of Science) up to March 2022 under the PRISMA guidelines. Forty-seven studies were screened using predefined inclusion and exclusion criteria. Finally, the current systematic review included nine publications that met the inclusion criteria.

Result: According to in vivo findings, melatonin treatment reduces DN via inhibition of oxidative stress and inflammatory pathways. However, compared to the diabetes groups alone, melatonin treatment exhibited an anti-oxidant trend. According to other research, DN also significantly produces biochemical alterations in neuron cells/tissues. Additionally, histological alterations in neuron tissue following DN were detected.

Conclusion: Nonetheless, in the majority of cases, these diabetes-induced biochemical and histological alterations were reversed when melatonin was administered. It is worth noting that the administration of melatonin ameliorates the neuropathy caused by diabetes. Melatonin exerts these neuroprotective effects via various anti-oxidant, anti-inflammatory, and other mechanisms.

Anti-Inflammatory Effects of Melatonin in Rats with Induced Type 2 Diabetes Mellitus

Introduction: Insulin resistance is associated with a pro-inflammatory state increasing the risk for complications in patients with type 2 diabetes mellitus (T2DM). In addition to its chronobiotic effects, the pineal hormone melatonin is known to exert anti-inflammatory and antioxidant effects. Melatonin was also suggested to affect insulin secretion. The aim of this study was therefore to investigate the effect of melatonin on inflammation in diabetic rats and to study the possible involvement of the melatonin receptor, MT2. Materials and Methods: Male Sprague Dawley rats were randomly divided into four experimental groups (n = 10 per group): (1) control, (2) streptozotocin/nicotinamide induced diabetes type 2 (T2DM), (3) T2DM treated with melatonin (500 µg/kg/day), and (4) T2DM treated with melatonin (500 µg/kg/day for 6 weeks) and the selective MT2 receptor antagonist luzindole (0.25 g/kg/day for 6 weeks). Blood samples were taken for biochemical parameters and various tissue samples (liver, adipose tissue, brain) were removed for immunohistochemistry (IHC), Western blot (WB), and Q-PCR analyses, respectively. Results: Melatonin significantly reduced increased blood levels of liver transaminases (AST, ALT), blood urea nitrogen (BUN), triglyceride, very low-density lipoprotein (VLDL), and cholesterol in diabetic rats with luzindole treatment partly reversing this effect regarding the lipids. Furthermore, the liver and adipose tissues of T2DM rats treated with melatonin showed lower expression of the inflammatory markers IL-1β, IL-6, TNF-α, and NF-κB as compared to the T2DM group without melatonin. The results also showed that the MT2 receptor is at least partly involved in the protective effects of melatonin. Conclusions: Our results suggest that melatonin exerts relevant anti-inflammatory effects on various tissues in type 2 diabetic rats.

Interplay of Dinner Timing and MTNR1B Type 2 Diabetes Risk Variant on Glucose Tolerance and Insulin Secretion: A Randomized Crossover Trial

OBJECTIVE

We tested whether the concurrence of food intake and elevated concentrations of endogenous melatonin, as occurs with late eating, results in impaired glucose control, in particular in carriers of the type 2 diabetes-associated G allele in the melatonin receptor-1B gene (MTNR1B).

RESEARCH DESIGN AND METHODS

In a Spanish natural late-eating population, a randomized, crossover study was performed. Each participant (n = 845) underwent two evening 2-h 75-g oral glucose tolerance tests following an 8-h fast: an early condition scheduled 4 h prior to habitual bedtime ("early dinner timing") and a late condition scheduled 1 h prior to habitual bedtime ("late dinner timing"), simulating an early and a late dinner timing, respectively. Differences in postprandial glucose and insulin responses between early and late dinner timing were determined using incremental area under the curve (AUC) calculated by the trapezoidal method.

RESULTS

Melatonin serum levels were 3.5-fold higher in the late versus early condition, with late dinner timing resulting in 6.7% lower insulin AUC and 8.3% higher glucose AUC. The effect of late eating impairing glucose tolerance was stronger in the MTNR1B G-allele carriers than in noncarriers. Genotype differences in glucose tolerance were attributed to reductions in β-cell function (P for interaction, Pint glucose area under the curve = 0.009, Pint corrected insulin response = 0.022, and Pint disposition index = 0.018).

CONCLUSIONS

Concurrently high endogenous melatonin and carbohydrate intake, as typical for late eating, impairs glucose tolerance, especially in MTNR1B G-risk allele carriers, attributable to insulin secretion defects.

Melatonin as the Cornerstone of Neuroimmunoendocrinology

Much attention has been recently drawn to studying melatonin – a hormone whose synthesis was first found in the epiphysis (pineal gland). This interest can be due to discovering the role of melatonin in numerous physiological processes. It was the discovery of melatonin synthesis in endocrine organs (pineal gland), neural structures (Purkinje cells in the cerebellum, retinal photoreceptors), and immunocompetent cells (T lymphocytes, NK cells, mast cells) that triggered the evolution of new approaches to the unifield signal regulation of homeostasis, which, at the turn of the 21st century, lead to the creation of a new integral biomedical discipline — neuroimmunoendocrinology. While numerous hormones have been verified over the last decade outside the “classical” locations of their formation, melatonin occupies an exclusive position with regard to the diversity of locations where it is synthesized and secreted. This review provides an overview and discussion of the major data regarding the role of melatonin in various physiological and pathological processes, which affords grounds for considering melatonin as the “cornerstone” on which neuroimmunoendocrinology has been built as an integral concept of homeostasis regulation.

Diabetes mellitus and melatonin: Where are we?

Diabetes mellitus (DM) and diabetes-related complications are amongst the leading causes of mortality worldwide. The international diabetes federation (IDF) has estimated 592 million people to suffer from DM by 2035. Hence, finding a novel biomolecule that can effectively aid diabetes management is vital, as other existing drugs have numerous side effects. Melatonin, a pineal hormone having antioxidative and anti-inflammatory properties, has been implicated in circadian dysrhythmia-linked DM. Reduced levels of melatonin and a functional link between melatonin and insulin are implicated in the pathogenesis of type 2 diabetes (T2D) Additionally, genomic studies revealed that rare variants in melatonin receptor 1b (MTNR1B) are also associated with impaired glucose tolerance and increased risk of T2D. Moreover, exogenous melatonin treatment in cell lines, rodent models, and diabetic patients has shown a potent effect in alleviating diabetes and other related complications. This highlights the role of melatonin in glucose homeostasis. However, there are also contradictory reports on the effects of melatonin supplementation. Thus, it is essential to explore if melatonin can be taken from bench to bedside for diabetes management. This review summarizes the therapeutic potential of melatonin in various diabetic models and whether it can be considered a safe drug for managing diabetic complications and diabetic manifestations like oxidative stress, inflammation, ER stress, mitochondrial dysfunction, metabolic dysregulation, etc.

Human and Rodent Cell Lines as Models of Functional Melatonin-Responsive Pancreatic Islet Cells

Cell culture of different pancreatic islet cell lines, like the murine α-cell line αTC1.9, the rat β-cell lines INS-1 and INS-1 832/13, and the human δ-cell line QGP-1, can serve as valuable cell models for the analysis of melatonin-dependent modulation of hormone secretion. The paper summarizes in detail the requirements of culture for each cell line and includes batch protocols to stimulate hormone secretion and to treat cells with several melatonin concentrations as previously published. We here describe the processing of collected cell pellets or cell culture supernatants as well as different methods to analyze cell experiments after melatonin treatment on the basis of our own experience. Finally, we outlined for each cell line under which conditions the melatonin treatment should be performed to gain reproducible results.

The rs10830963 Polymorphism of the MTNR1B Gene: Association With Abnormal Glucose, Insulin and C-peptide Kinetics

BACKGROUND

The MTNR1B gene encodes a receptor for melatonin, a hormone regulating biorhythms. Disruptions in biorhythms contribute to the development of type 2 diabetes mellitus (T2DM). Genetic studies suggest that variability in the MTNR1B gene affects T2DM development. Our aim was to compare the distribution of the genetic variant rs10830963 between persons differing in glucose tolerance in a sample of the Czech population (N=1206). We also evaluated possible associations of the polymorphism with insulin sensitivity, beta cell function, with the shape of glucose, insulin and C-peptide trajectories measured 7 times during a 3-hour oral glucose tolerance test (OGTT) and with glucagon response. In a subgroup of 268 volunteers we also evaluated sleep patterns and biorhythm.

RESULTS

13 persons were diagnosed with T2DM, 119 had impaired fasting blood glucose (IFG) and/or impaired glucose tolerance (IGT). 1074 participants showed normal results and formed a control group. A higher frequency of minor allele G was found in the IFG/IGT group in comparison with controls. The GG constellation was present in 23% of diabetics, in 17% of IFG/IGT probands and in 11% of controls. Compared to CC and CG genotypes, GG homozygotes showed higher stimulated glycemia levels during the OGTT. Homozygous as well as heterozygous carriers of the G allele showed lower very early phase of insulin and C-peptide secretion with unchanged insulin sensitivity. These differences remained significant after excluding diabetics and the IFG/IGT group from the analysis. No associations of the genotype with the shape of OGTT-based trajectories, with glucagon or with chronobiological patterns were observed. However, the shape of the trajectories differed significantly between men and women.

CONCLUSION

In a representative sample of the Czech population, the G allele of the rs10830963 polymorphism is associated with impaired early phase of beta cell function, and this is evident even in healthy individuals.

Membrane Melatonin Receptors Activated Cell Signaling in Physiology and Disease

The pineal hormone melatonin has attracted great scientific interest since its discovery in 1958. Despite the enormous number of basic and clinical studies the exact role of melatonin in respect to human physiology remains elusive. In humans, two high-affinity receptors for melatonin, MT1 and MT2, belonging to the family of G protein-coupled receptors (GPCRs) have been cloned and identified. The two receptor types activate Gi proteins and MT2 couples additionally to Gq proteins to modulate intracellular events. The individual effects of MT1 and MT2 receptor activation in a variety of cells are complemented by their ability to form homo- and heterodimers, the functional relevance of which is yet to be confirmed. Recently, several melatonin receptor genetic polymorphisms were discovered and implicated in pathology-for instance in type 2 diabetes, autoimmune disease, and cancer. The circadian patterns of melatonin secretion, its pleiotropic effects depending on cell type and condition, and the already demonstrated cross-talks of melatonin receptors with other signal transduction pathways further contribute to the perplexity of research on the role of the pineal hormone in humans. In this review we try to summarize the current knowledge on the membrane melatonin receptor activated cell signaling in physiology and pathology and their relevance to certain disease conditions including cancer.

The Effect of Melatonin on Incretin Hormones: Results From Experimental and Randomized Clinical Studies

CONTEXT

Glucose homeostasis is under circadian control through both endocrine and intracellular mechanisms, with several lines of evidence suggesting that melatonin affects glucose homeostasis.

OBJECTIVE

To evaluate the acute in vivo and in situ effects of melatonin on secretion of the incretin hormones, glucagon-like-peptide 1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP), and their impact on β-cell insulin secretion.

DESIGN

A human randomized, double-blinded, placebo-controlled crossover study combined with a confirmatory in situ study of perfused rat intestines.

SETTING

Aarhus University Hospital.

METHODS

Fifteen healthy male participants were examined 2 × 2 times: an oral glucose tolerance test (OGTT) was performed on day 1 and an isoglycemic IV glucose infusion replicating the blood glucose profile of the OGTT day was performed on day 2. These pairs of study days were repeated on treatment with melatonin and placebo, respectively. For the in situ study, 6 rat intestines and 4 rat pancreases were perfused arterially with perfusion buffer ± melatonin. The intestines were concomitantly perfused with glucose through the luminal compartment.

RESULTS

In humans, melatonin treatment resulted in reduced GIP secretion compared with placebo (ANOVA P = 0.003), an effect also observed in the perfused rat intestines (ANOVA P = 0.003), in which GLP-1 secretion also was impaired by arterial melatonin infusion (ANOVA P < 0.001). Despite a decrease in GIP levels, the in vivo glucose-stimulated insulin secretion was unaffected by melatonin (P = 0.78).

CONCLUSION

Melatonin reduced GIP secretion during an oral glucose challenge in healthy young men but did not affect insulin secretion. Reduced GIP secretion was confirmed in an in situ model of the rat intestine.

Effects of Melatonin Supplementation on Insulin Levels and Insulin Resistance: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

Insulin resistance (IR) is a pivotal process in various metabolic diseases. The well-known treatment is lifestyle modification and medication therapy, which may result in poor compliance and side effects. Melatonin has been suggested to have a role in glucose metabolism, yet the results across studies have been inconsistent. Therefore, we performed a systematic review to evaluate the effects of melatonin supplementation on insulin levels and IR. We searched PubMed, Embase, Cochrane Central Register of Controlled Trials, and ClinicalTrials.gov, and identified randomized controlled trials (RCTs) published prior to August 2020. Articles were reviewed, selected and extracted by two reviewers independently. In total, 8 RCTs of 376 participants were included. Data were pooled using a random-effects model, with mean differences (MDs) and 95% confidence intervals (CIs). Our results showed that melatonin administration significantly reduced insulin levels and homeostasis model assessment of insulin resistance (HOMA-IR), and increased the quantitative insulin sensitivity check index (QUICKI). We conclude that melatonin ameliorated hyperinsulinemia, insulin resistance, and insulin sensitivity, and the results are an update of a previous meta-analysis. Although more investigations are required, we clearly provide evidence for the use of melatonin as an adjuvant treatment for metabolic disorders involving IR.

Acute metabolic effects of melatonin-A randomized crossover study in healthy young men

Melatonin regulates circadian rhythm, but may also have effects on glucose homeostasis. A common G-allele in the MTNR1B locus has been associated with an increased risk of type 2 diabetes (T2DM). We aimed to examine acute effects of high doses of melatonin on glucose metabolism with attention to MTNR1B genotype. Twenty men were examined in a double-blinded, randomized crossover study on two nonconsecutive days with four doses of 10 mg oral melatonin or placebo. Insulin sensitivity and insulin secretion were assessed by an intravenous glucose tolerance test (IVGTT) and a hyperinsulinaemic-euglycaemic clamp (HEC). Blood samples were drawn to determine the metabolic profile and MTNR1B rs10830963 genotype. Indirect calorimetry and blood pressure measurements were also performed. Insulin sensitivity index was significantly reduced on the melatonin day (P = .028) in the whole group and in homozygous carriers of the rs10830963 C-allele (P = .041). Glucose during the IVGTT was unaffected, but there was a tendency towards lower insulin and C-peptide levels in the first minutes after glucose administration in G-allele carriers. Systolic blood pressure decreased and lipid oxidation increased significantly on the melatonin day in rs10830963 G-allele carriers. Overall, our study reports that acute administration of melatonin in supra-physiological doses may have a negative impact on insulin sensitivity. Clinical trial registration number (clinicaltrial.gov): NCT03204877.

Maternal Melatonin Deficiency Leads to Endocrine Pathologies in Children in Early Ontogenesis

The review summarizes the results of experimental and clinical studies aimed at elucidating the causes and pathophysiological mechanisms of the development of endocrine pathology in children. The modern data on the role of epigenetic influences in the early ontogenesis of unfavorable factors that violate the patterns of the formation of regulatory mechanisms during periods of critical development of fetal organs and systems and contribute to the delayed development of pathological conditions are considered. The mechanisms of the participation of melatonin in the regulation of metabolic processes and the key role of maternal melatonin in the formation of the circadian system of regulation in the fetus and in the protection of the genetic program of its morphofunctional development during pregnancy complications are presented. Melatonin, by controlling DNA methylation and histone modification, prevents changes in gene expression that are directly related to the programming of endocrine pathology in offspring. Deficiency and absence of the circadian rhythm of maternal melatonin underlies violations of the genetic program for the development of hormonal and metabolic regulatory mechanisms of the functional systems of the child, which determines the programming and implementation of endocrine pathology in early ontogenesis, contributing to its development in later life. The significance of this factor in the pathophysiological mechanisms of endocrine disorders determines a new approach to risk assessment and timely prevention of offspring diseases even at the stage of family planning.

Relationship Between Melatonin and Cardiovascular Disease

Coronary artery disease (CAD) is one of the leading causes of morbidity and mortality worldwide. The coronary atherosclerotic process involves different pathological mechanisms; inflammation is one of the major triggers for the development of atherosclerotic plaque. Although several studies showed the favorable effects of melatonin on the cardiovascular system (CVS), melatonin seems not to take its rightful place in today's clinical practice. This review aims to point out the role of melatonin on cardiovascular disease (CVD) and its' risk factors. All data were obtained via PubMed, Wikipedia, and Google.

Integrated Metabolomics and Lipidomics Analysis Reveal Remodeling of Lipid Metabolism and Amino Acid Metabolism in Glucagon Receptor-Deficient Zebrafish

The glucagon receptor (GCGR) is activated by glucagon and is essential for glucose, amino acid, and lipid metabolism of animals. GCGR blockade has been demonstrated to induce hypoglycemia, hyperaminoacidemia, hyperglucagonemia, decreased adiposity, hepatosteatosis, and pancreatic α cells hyperplasia in organisms. However, the mechanism of how GCGR regulates these physiological functions is not yet very clear. In our previous study, we revealed that GCGR regulated metabolic network at transcriptional level by RNA-seq using GCGR mutant zebrafish (gcgr ^-/-). Here, we further performed whole-organism metabolomics and lipidomics profiling on wild-type and gcgr ^-/- zebrafish to study the changes of metabolites. We found 107 significantly different metabolites from metabolomics analysis and 87 significantly different lipids from lipidomics analysis. Chemical substance classification and pathway analysis integrated with transcriptomics data both revealed that amino acid metabolism and lipid metabolism were remodeled in gcgr-deficient zebrafish. Similar to other studies, our study showed that gcgr ^-/- zebrafish exhibited decreased ureagenesis and impaired cholesterol metabolism. More interestingly, we found that the glycerophospholipid metabolism was disrupted, the arachidonic acid metabolism was up-regulated, and the tryptophan metabolism pathway was down-regulated in gcgr ^-/- zebrafish. Based on the omics data, we further validated our findings by revealing that gcgr ^-/- zebrafish exhibited dampened melatonin diel rhythmicity and increased locomotor activity. These global omics data provide us a better understanding about the role of GCGR in regulating metabolic network and new insight into GCGR physiological functions.

A functional polymorphism rs10830963 in melatonin receptor 1B associated with the risk of gestational diabetes mellitus

The melatonin receptor 1B (MTNR1B) polymorphism rs10830963 C>G has been reported to be associated with the risk of gestational diabetes mellitus (GDM) with inconsistent results. To clarify the effect of the polymorphism on the risk of GDM, a meta-analysis therefore was performed. Pooled OR with its corresponding 95%CI was used to estimate the strength of the association. Totally 14 eligible studies with a number of 5033 GDM patients and 5614 controls were included in this meta-analysis. Results indicated that the variant G allele was significantly associated with an increased GDM risk (CG vs. CC: OR = 1.25, 95% CI = 1.11−1.40, P < 0.001; GG vs. CC: OR = 1.78, 95% CI = 1.45−2.19, P < 0.001; G vs. C: OR = 1.33, 95% CI = 1.21−1.47, P < 0.001). In the stratified analysis by ethnicity, similar results were found in Asians (CG vs. CC: OR = 1.15, 95%CI = 1.02−1.28, P = 0.020; GG vs. CC: OR = 1.52, 95% CI = 1.23−1.89, P < 0.001; G vs. C: OR = 1.23, 95% CI = 1.10−1.37, P < 0.001) and in Caucasians (CG vs. CC: OR = 1.40, 95% CI = 1.16−1.70, P < 0.001; GG vs. CC: OR = 2.21, 95% CI = 1.54−3.17, P < 0.001; G vs. C: OR = 1.47, 95% CI = 1.24−1.73, P < 0.001). FPRP and TSA analyses confirmed findings support that the rs10830963 G allele increases the risk of GDM, and further functional experimental studies are warranted to explore and clarify the potential mechanism.

Mechanisms of the Regulation and Dysregulation of Glucagon Secretion

Glucagon, a hormone secreted by pancreatic alpha cells, contributes to the maintenance of normal blood glucose concentration by inducing hepatic glucose production in response to declining blood glucose. However, glucagon hypersecretion contributes to the pathogenesis of type 2 diabetes. Moreover, diabetes is associated with relative glucagon undersecretion at low blood glucose and oversecretion at normal and high blood glucose. The mechanisms of such alpha cell dysfunctions are not well understood. This article reviews the genesis of alpha cell dysfunctions during the pathogenesis of type 2 diabetes and after the onset of type 1 and type 2 diabetes. It unravels a signaling pathway that contributes to glucose- or hydrogen peroxide-induced glucagon secretion, whose overstimulation contributes to glucagon dysregulation, partly through oxidative stress and reduced ATP synthesis. The signaling pathway involves phosphatidylinositol-3-kinase, protein kinase B, protein kinase C delta, non-receptor tyrosine kinase Src, and phospholipase C gamma-1. This knowledge will be useful in the design of new antidiabetic agents or regimens.

Melatonin protects INS-1 pancreatic β-cells from apoptosis and senescence induced by glucotoxicity and glucolipotoxicity

INTRODUCTION

Melatonin is a hormone known as having very strong anti-oxidant property. Senescence is a biological state characterized by the loss of cell replication and the changes consisting of a pro-inflammatory phenotype, leading to Senescence Associated Secretory Phenotype (SASP) which is now regarded as one of the fundamental processes of many degenerative diseases. Increased cell division count induces cell senescence via DNA damage in response to elevated Reactive Oxygen Species (ROS). We wanted to test whether melatonin could reduce apoptosis and stress induced premature pancreatic β-cell senescence induced by glucotoxicity and glucolipotoxicity.

MATERIALS AND METHOD

Cultured rodent pancreatic β-cell line (INS-1 cell) was used. Glucotoxicity (HG: hyperglycemia) and glucolipotoxicity (HGP: hyperglycemia with palmitate) were induced by hyperglycemia and the addition of palmitate. The degrees of the senescence were measured by SA-β-Gal and P16^lnk4A staining along with the changes of cell viabilities, cell cycle-related protein and gene expressions, endogenous anti-oxidant defense enzymes, and Glucose Stimulated Insulin Secretion (GSIS), before and after melatonin treatment.

RESULTS

Cultured INS-1 cells in HG and HGP conditions revealed accelerated senescence, increased apoptosis, cell cycle arrest, compromised endogenous anti-oxidant defense, and impaired glucose-stimulated insulin secretion. Melatonin decreased apoptosis and expressions of proteins related to senescence, increase the endogenous anti-oxidant defense, and improved glucose-stimulated insulin secretion.

CONCLUSION

Melatonin protected pancreatic β-cell from apoptosis, decreased expressions of the markers related to the accelerated senescence, and improved the biological deteriorations induced by glucotoxicity and glucolipotoxicity.

Effects of altered photoperiod due to COVID-19 lockdown on pregnant women and their fetuses

Maternal circadian rhythms provide highly important input into the entrainment and programming of fetal and newborn circadian rhythms. The light-dark cycle is an important regulator of the internal biological clock. Even though pregnant women spend a greater part of the day at home during the latter stages of pregnancy, natural light exposure is crucial for the fetus. The current recommended COVID-19 lockdown might dramatically alter normal environmental lighting conditions of pregnant women, resulting in exposure to extremely low levels of natural daylight and high-intensity artificial light sources during both day and night. This article summarizes the potential effects on pregnant woman and their fetuses due to prolonged exposure to altered photoperiod and as consequence altered circadian system, known as chronodisruption, that may result from the COVID-19 lockdown.

Insights into pancreatic islet cell dysfunction from type 2 diabetes mellitus genetics

Type 2 diabetes mellitus (T2DM) is an increasingly prevalent multifactorial disease that has both genetic and environmental risk factors, resulting in impaired glucose homeostasis. Genome-wide association studies (GWAS) have identified over 400 genetic signals that are associated with altered risk of T2DM. Human physiology and epigenomic data support a central role for the pancreatic islet in the pathogenesis of T2DM. This Review focuses on the promises and challenges of moving from genetic associations to molecular mechanisms and highlights efforts to identify the causal variant and effector transcripts at T2DM GWAS susceptibility loci. In addition, we examine current human models that are used to study both β-cell development and function, including EndoC-β cell lines and human induced pluripotent stem cell-derived β-like cells. We use examples of four T2DM susceptibility loci (CDKAL1, MTNR1B, SLC30A8 and PAM) to emphasize how a holistic approach involving genetics, physiology, and cellular and developmental biology can disentangle disease mechanisms at T2DM GWAS signals.

Melatonin Effects on Glucose Metabolism: Time To Unlock the Controversy

The past decade has witnessed a revival of interest in the hormone melatonin, partly attributable to the discovery that genetic variation in MTNR1B - the melatonin receptor gene - is a risk factor for impaired fasting glucose and type 2 diabetes (T2D). Despite intensive investigation, there is considerable confusion and seemingly conflicting data on the metabolic effects of melatonin and MTNR1B variation, and disagreement on whether melatonin is metabolically beneficial or deleterious, a crucial issue for melatonin agonist/antagonist drug development and dosing time. We provide a conceptual framework - anchored in the dimension of 'time' - to reconcile paradoxical findings in the literature. We propose that the relative timing between elevated melatonin concentrations and glycemic challenge should be considered to better understand the mechanisms and therapeutic opportunities of melatonin signaling in glycemic health and disease.

Combined administration of exogenous melatonin and insulin ameliorates streptozotocin induced toxic alteration on hematological parameters in diabetic male Wistar rats

The aim of the present was to ameliorate the protective effect of exogenous melatonin and insulin against the diabetes induced alterations in the different hematological variables. Albino rats were administrated streptozotocin at the dose of 15 mg/kg for 6 days. Total 54 rats were randomly selected for the experimental purpose and were divided into two major groups. Group-1 consisting twenty four (24) and were further sub-divided into four (4) different groups viz. group-I served as normal control, group-II served as melatonin treated, group-III served as insulin treated and group-IV served as glibenclamide treated. Group-2 consisting thirty (30) rats were given streptozotocin (STZ) injection (15 mg/kg) for 6 days. After confirmation of diabetes by measuring blood glucose level, animals having blood glucose level above 250 mg/dl) confirmed as diabetic. Thirty (30) Diabetic rats were further subdivided into following sub-groups and were given different therapeutic treatments, Viz group-I served as Diabetic control, group-II treated with melatonin, group-III treated with insulin, group-IV given treatment of melatonin and insulin and group-V were given treatment of glibenclamide respectively. Diabetic rats showed modulation in all the studied hematological variables. Diabetic rats displayed significant decline in RBCs count, HB level and its associated indices (HCT, RDW, MCV, MCH, MCHC), WBCs and its related indices (polymorphs and lymphocytes) and platelet distribution width (PDW %) whereas platelet count showed significant increase. Nonetheless alone as well as combined treatment of exogenous melatonin and insulin restored all altered hematological parameters. However, significant recovery was found in the group in which combined dose of melatonin and insulin was administrated. Therefore, it might be concluded that combined administration of melatonin and insulin will be better remedy to normalize the altered blood profile during the diabetic condition.

Melatonin and Oxidative Stress in the Diabetic State: Clinical Implications and Potential Therapeutic Applications

All living organisms exhibit circadian rhythms, which govern the majority of biological functions, including metabolic processes. Misalignment of these circadian rhythms increases the risk of developing metabolic diseases. Thus, disruption of the circadian system has been proven to affect the onset of type 2 diabetes mellitus (T2DM). In this context, the pineal indoleamine melatonin is a signaling molecule able to entrain circadian rhythms. There is mounting evidence that suggests a link between disturbances in melatonin production and impaired insulin, glucose, lipid metabolism, and antioxidant capacity. Besides, several genetic association studies have causally associated various single nucleotide polymorphysms (SNPs) of the human MT2 receptor with increased risk of developing T2DM. Taken together, these data suggest that endogenous as well as exogenous melatonin may influence diabetes and associated metabolic disturbances not only by regulating insulin secretion but also by providing protection against reactive oxygen species (ROS) since pancreatic β-cells are very susceptible to oxidative stress due to their low antioxidant capacity.

Reduced melatonin synthesis in pregnant night workers: Metabolic implications for offspring

Several novel animal studies have shown that intrauterine metabolic programming can be modified in the event of reduced melatonin synthesis during pregnancy, leading to glucose intolerance and insulin resistance in the offspring. It is therefore postulated that female night workers when pregnant may expose the offspring to unwanted health threats. This may be explained by the fact that melatonin is essential for regulating energy metabolism and can influence reproductive activity. Moreover, the circadian misalignment caused by shift work affects fertility and the fetus, increasing the risk of miscarriage, premature birth and low birth weight, phenomena observed in night workers. Thus, we hypothesize that light-induced melatonin suppression as a result of night work may alter intrauterine metabolic programming in pregnant women, potentially leading to metabolic disorders in their offspring.

Effects of Ramelteon and Other Sleep-Promoting Drugs on Serum Low-Density Lipoprotein and Non-high-density Lipoprotein Cholesterol: A Retrospective Comparative Pilot Study

Melatonin has been suggested to play important roles in lipid metabolism as well as circadian rhythm; however, very few studies explored the effects of ramelteon, a selective melatonin receptor agonist, on serum lipid profiles. In this study effects of ramelteon on serum lipid profiles were explored, comparing to those of other sleep-promoting drugs including benzodiazepines and non-benzodiazepines, in patients with insomnia. We retrospectively reviewed medical charts of outpatients who were treated with ramelteon (8 mg/d) or other sleep-promoting drugs for no less than 8 weeks during the period between October 1st, 2011 and September 30th, 2014, and compared the changes in serum lipid profiles between the two groups. Patients with regular dialysis or malignant diseases treated with cytotoxic anti-cancer drugs, or whose lipid-lowering drugs were altered during the study period, were excluded. Among 365 or 855 outpatients treated with ramelteon or other sleep-promoting drugs, 35 or 46 patients, respectively, had complete serum low-density lipoprotein cholesterol (LDL-C) or non-high-density lipoprotein cholesterol (non-HDL-C) data. Serum LDL-C was significantly reduced from 103.1±4.4 to 94.6±4.2 mg/dL (8.2% reduction, p<0.05, n=31) in the ramelteon group, and was not significantly changed (p=0.23, n=40) in the other sleep-promoting drug group. Non-HDL-C was significantly decreased from 138.8±6.0 to 130.6±4.9 mg/dL (5.9% reduction, p<0.05, n=32) in the ramelteon group, and was not significantly altered (p=0.29, n=42) in the other sleep-promoting drug group. Ramelteon, but not other sleep-promoting drugs, specifically lowers serum LDL-C and non-HDL-C levels.

Detection of recombinant and endogenous mouse melatonin receptors by monoclonal antibodies targeting the C-terminal domain

Melatonin receptors play important roles in the regulation of circadian and seasonal rhythms, sleep, retinal functions, the immune system, depression, and type 2 diabetes development. Melatonin receptors are approved drug targets for insomnia, non-24-hour sleep-wake disorders, and major depressive disorders. In mammals, two melatonin receptors (MTRs) exist, MT₁ and MT₂ , belonging to the G protein-coupled receptor (GPCR) superfamily. Similar to most other GPCRs, reliable antibodies recognizing melatonin receptors proved to be difficult to obtain. Here, we describe the development of the first monoclonal antibodies (mABs) for mouse MT₁ and MT₂ . Purified antibodies were extensively characterized for specific reactivity with mouse, rat, and human MT₁ and MT₂ by Western blot, immunoprecipitation, immunofluorescence, and proximity ligation assay. Several mABs were specific for either mouse MT₁ or MT₂ . None of the mABs cross-reacted with rat MTRs, and some were able to react with human MTRs. The specificity of the selected mABs was validated by immunofluorescence microscopy in three established locations (retina, suprachiasmatic nuclei, pituitary gland) for MTR expression in mice using MTR-KO mice as control. MT₂ expression was not detected in mouse insulinoma MIN6 cells or pancreatic beta-cells. Collectively, we report the first monoclonal antibodies recognizing recombinant and native mouse melatonin receptors that will be valuable tools for future studies.

Melatonin in type 2 diabetes mellitus and obesity

Despite considerable advances in the past few years, obesity and type 2 diabetes mellitus (T2DM) remain two major challenges for public health systems globally. In the past 9 years, genome-wide association studies (GWAS) have established a major role for genetic variation within the MTNR1B locus in regulating fasting plasma levels of glucose and in affecting the risk of T2DM. This discovery generated a major interest in the melatonergic system, in particular the melatonin MT₂ receptor (which is encoded by MTNR1B). In this Review, we discuss the effect of melatonin and its receptors on glucose homeostasis, obesity and T2DM. Preclinical and clinical post-GWAS evidence of frequent and rare variants of the MTNR1B locus confirmed its importance in regulating glucose homeostasis and T2DM risk with minor effects on obesity. However, these studies did not solve the question of whether melatonin is beneficial or detrimental, an issue that will be discussed in the context of the peculiarities of the melatonergic system. Melatonin receptors might have therapeutic potential as they belong to the highly druggable G protein-coupled receptor superfamily. Clarifying the precise role of melatonin and its receptors on glucose homeostasis is urgent, as melatonin is widely used for other indications, either as a prescribed medication or as a supplement without medical prescription, in many countries in Europe and in the USA.

Circadian clocks and insulin resistance

Insulin resistance is a main determinant in the development of type 2 diabetes mellitus and a major cause of morbidity and mortality. The circadian timing system consists of a central brain clock in the hypothalamic suprachiasmatic nucleus and various peripheral tissue clocks. The circadian timing system is responsible for the coordination of many daily processes, including the daily rhythm in human glucose metabolism. The central clock regulates food intake, energy expenditure and whole-body insulin sensitivity, and these actions are further fine-tuned by local peripheral clocks. For instance, the peripheral clock in the gut regulates glucose absorption, peripheral clocks in muscle, adipose tissue and liver regulate local insulin sensitivity, and the peripheral clock in the pancreas regulates insulin secretion. Misalignment between different components of the circadian timing system and daily rhythms of sleep-wake behaviour or food intake as a result of genetic, environmental or behavioural factors might be an important contributor to the development of insulin resistance. Specifically, clock gene mutations, exposure to artificial light-dark cycles, disturbed sleep, shift work and social jet lag are factors that might contribute to circadian disruption. Here, we review the physiological links between circadian clocks, glucose metabolism and insulin sensitivity, and present current evidence for a relationship between circadian disruption and insulin resistance. We conclude by proposing several strategies that aim to use chronobiological knowledge to improve human metabolic health.

GPCRs in Autocrine and Paracrine Regulations

G protein-coupled receptors (GPCRs) constitute the largest superfamily of integral membrane protein receptors. As signal detectors, the several 100 known GPCRs are responsible for sensing the plethora of endogenous ligands that are critical for the functioning of our endocrine system. Although GPCRs are typically considered as detectors for first messengers in classical signal transduction pathways, they seldom operate in isolation in complex biological systems. Intercellular communication between identical or different cell types is often mediated by autocrine or paracrine signals that are generated upon activation of specific GPCRs. In the context of energy homeostasis, the distinct complement of GPCRs in each cell type bridges the autocrine and paracrine communication within an organ, and the various downstream signaling mechanisms regulated by GPCRs can be integrated in a cell to produce an ultimate output. GPCRs thus act as gatekeepers that coordinate and fine-tune a response. By examining the role of GPCRs in activating and receiving autocrine and paracrine signals, one may have a better understanding of endocrine diseases that are associated with GPCR mutations, thereby providing new insights for treatment regimes.

Is a Combination of Melatonin and Amino Acids Useful to Sarcopenic Elderly Patients? A Randomized Trial

This study evaluated the effectiveness of a 4-week intervention of melatonin and essential aminoacid supplementation on body composition, protein metabolism, strength and inflammation in 159 elderly sarcopenic patients (42/117, men/women), assigned to four groups: isocaloric placebo (P, n = 44), melatonin (M, 1 mg/daily, n = 42), essential amino acids (eAA 4 g/daily, n = 40) or eAA plus melatonin (eAAM, 4 g eAA and 1 mg melatonin/daily, n = 30). Data from body composition (dual X-ray absortiometry (DXA)), strength (handgrip test) and biochemical parameters for the assessment of protein metabolism (albumin) and inflammation (CRP) were collected at baseline and after the 4-week intervention. Compared with P and M, supplementation with eAA plus M increased total fat-free mass (vs. P: +2190 g; p < 0.01; vs. M: +2107 g; p < 0.05). M alone lowered albumin levels (vs. P: −0.39 g; p < 0.01; vs. eAA: −0.47 g; p < 0.01). This data on albumin was confirmed by within-group analysis (M −0.44g; p < 0.001; eAAM: −0.34 p < 0.05). M and eAA seemed to lower the percentage of gynoid fat (p < 0.05) and android fat (p < 0.01). No significant changes in inflammation or strength were reported. A 4-week intervention with eAA plus M together may be effective in enhancing fat-free-mass compared to M and P but not versus eAA. M alone demonstrates a negative effect on albumin level.

Gender differences in the association between melatonin secretion and diabetes in elderly: The HEIJO-KYO cohort

OBJECTIVE

Reportedly, melatonin protects the pancreatic islets and decreases insulin resistance; thus, it may contribute to preventing diabetes. Epidemiological data suggested that lower melatonin secretion is associated with higher incidence of diabetes in female nurses. Such associations are unknown in the general population. We evaluated the association between melatonin secretion and diabetes in a general population, including both genders.

DESIGN

Cross-sectional study.

PARTICIPANTS

A total of 1096 community-based elderly males (n = 519) and females (n = 577) (mean age, 71.8 years) were enrolled.

MEASUREMENTS

Overnight urinary 6-sulfatoxymelatonin excretion (UME) and diabetes prevalence were measured.

RESULTS

The median UME was 6.7 μg (interquartile range, 4.0-10.5); the prevalence of diabetes was 17.5% in males and 10.7% in females. The prevalence of diabetes decreased with increasing UME quartiles among males (P for trend = 0.009) but not among females (P for trend = 0.96). In a multivariable logistic regression analysis adjusting for potential confounding factors, such as age, smoking and drinking habits, economic status, caloric intake, and physical activity, the odds ratio (OR) for diabetes was significantly lower in the highest UME quartile group compared with the lowest quartile group among males (OR, 0.35; 95% CI, 0.17-0.70; P = 0.003) but not females (OR, 0.94; 95% CI, 0.45-1.95; P = 0.87). Consistent results were observed in the analysis after adjusting for clinical parameters or using continuous UME data.

CONCLUSIONS

Melatonin secretion was significantly and inversely associated with diabetes in males but not in females. This association was independent of several important confounding factors.

Melatonin improves hyperglycemia induced damages in rat brain

BACKGROUND

Diabetes mellitus is an endocrine disorder which is characterized by the development of resistance to the cellular activity of insulin or inadequate insulin production. It leads to hyperglycemia, prolonged inflammation, and oxidative stress. Oxidative stress is assumed to play an important role in the development of diabetic complications. Melatonin is the hormone that interacts with insulin in diabetes. Therefore, in this study, the effects of melatonin treatment with or without insulin were examined in diabetic rat brain.

METHODS

Rats were divided into five groups as control, diabetes, diabetes + insulin, diabetes + melatonin, and diabetes + melatonin + insulin. Experimental diabetes was induced by streptozotocin (60 mg/kg, i.p.). Twelve weeks after diabetes induction, rats were decapitated. Malondialdehyde, glutathione, sialic acid and nitric oxide levels, superoxide dismutase, catalase, glutathione-S-transferase, myeloperoxidase, and tissue factor activities were determined in brain tissue.

RESULTS

Melatonin alone showed its antioxidant effect by increasing brain glutathione level, superoxide dismutase, catalase, and glutathione-S-transferase activities and decreasing malondialdehyde level in experimental diabetes. Although insulin did not have a significant effect on glutathione and glutathione-S-transferase, its effects on lipid peroxidation, superoxide dismutase, and catalase were similar to melatonin; insulin also decreased myolopeoxidase activity and increased tissue factor activity. Combined melatonin and insulin treatment mimicked the effects of insulin.

CONCLUSION

Addition of melatonin to the insulin treatment did not change the effects of insulin, but the detailed role of melatonin alone in the treatment of diabetes merits further experimental and clinical investigation.

Effects of Streptozotocin-Induced Diabetes on the Pineal Gland in the Domestic Pig

Several observations from experiments in rodents and human patients suggest that diabetes affects pineal gland function, including melatonin secretion; however, the accumulated data are not consistent. The aim of the present study was to determine the effects of streptozotocin-induced diabetes on the pineal gland in the domestic pig, a species widely used as a model in various biomedical studies. The study was performed on 10 juvenile pigs, which were divided into two groups: control and diabetic. Diabetes was evoked by administration of streptozotocin (150 mg/kg of body weight). After six weeks, the animals were euthanized between 12.00 and 14.00, and the pineal glands were removed and divided into two equal parts, which were used for biochemical analyses and for preparation of explants for the superfusion culture. The pineal contents (per 100 μg protein) of serotonin, 5-hydroxyindole acetic acid, 5-hydroxytryptophol, 5-methoxyindole acetic acid, 5-methoxytryptophol, and 5-methoxytryptamine were significantly lower in diabetic pigs than in control pigs. In contrast, the level of N-acetylserotonin was significantly higher in diabetic animals. No significant differences were found in the level of melatonin between control and experimental pigs. The amounts of 3,4-dihydroxyphenylalanine, dopamine, norepinephrine, and 3,4-dihydroxyphenylacetic acid were significantly lower in the pineal glands of diabetic animals. The level of vanillylmandelic acid was higher in diabetic pigs. No differences were observed in the level of basal and NE-stimulated release of N-acetylserotonin or melatonin between the pineal explants prepared from control and experimental animals. In vitro treatment with insulin was ineffective. In conclusion, streptozotocin-induced diabetes affects both indole metabolism and adrenergic neurotransmission in the pig pineal gland.

Melatonin and pancreatic cancer: Current knowledge and future perspectives

Pancreatic cancer has a high mortality rate due to the absence of early symptoms and subsequent late diagnosis; additionally, pancreatic cancer has a high resistance to radio- and chemotherapy. Multiple inflammatory pathways are involved in the pathophysiology of pancreatic cancer. Melatonin an indoleamine produced in the pineal gland mediated and receptor-independent action is the pancreas and other where has both receptors. Melatonin is a potent antioxidant and tissue protector against inflammation and oxidative stress. In vivo and in vitro studies have shown that melatonin supplementation is an appropriate therapeutic approach for pancreatic cancer. Melatonin may be an effective apoptosis inducer in cancer cells through regulation of a large number of molecular pathways including oxidative stress, heat shock proteins, and vascular endothelial growth factor. Limited clinical studies, however, have evaluated the role of melatonin in pancreatic cancer. This review summarizes what is known regarding the effects of melatonin on pancreatic cancer and the mechanisms involved.

Type 2 diabetes-associated variants of the MT2 melatonin receptor affect distinct modes of signaling

Melatonin is produced during the night and regulates sleep and circadian rhythms. Loss-of-function variants in MTNR1B, which encodes the melatonin receptor MT₂, a G protein-coupled receptor (GPCR), are associated with an increased risk of type 2 diabetes (T2D). To identify specific T2D-associated signaling pathway(s), we profiled the signaling output of 40 MT₂ variants by monitoring spontaneous (ligand-independent) and melatonin-induced activation of multiple signaling effectors. Genetic association analysis showed that defects in the melatonin-induced activation of Gα_i1 and Gα_z proteins and in spontaneous β-arrestin2 recruitment to MT₂ were the most statistically significantly associated with an increased T2D risk. Computational variant impact prediction by in silico evolutionary lineage analysis strongly correlated with the measured phenotypic effect of each variant, providing a predictive tool for future studies on GPCR variants. Together, this large-scale functional study provides an operational framework for the postgenomic analysis of the multiple GPCR variants present in the human population. The association of T2D risk with signaling pathway-specific defects opens avenues for pathway-specific personalized therapeutic intervention and reveals the potential relevance of MT₂ function during the day, when melatonin is undetectable, but spontaneous activity of the receptor occurs.

Distribution and density of melatonin receptors in human main pancreatic islet cell types

Recent investigations of our group established that melatonin modulates hormone secretion of pancreatic islets via melatonin receptor types MT1 and MT2. Expression of MT1 and MT2 has been shown in mouse, rat, and human pancreatic islets as well as in the β-, α-, and δ-cell lines INS-1, αTC1.9, and QGP-1. In view of these earlier investigations, this study was performed to analyze in detail the distribution and density of melatonin receptors on the main islet cell types in human pancreatic tissue obtained from nondiabetic and type 2 diabetic patients. Immunohistochemical analysis established the presence of MT1 and MT2 in β-, α-, and δ-cells, but notably, with differences in receptor density. In general, the lowest MT1 and MT2 receptor density was measured in α-cells compared to the 2 other cell types. In type 2 diabetic islets, MT1 and MT2 receptor density was increased in δ-cells compared to normoglycemic controls. In human islets in batch culture of a nondiabetic donor, an increase of somatostatin secretion was observed under melatonin treatment while in islets of a type 2 diabetic donor, an inhibitory influence could be observed, especially in the presence of 5.5 mmol/L glucose. These data suggest the following: i) cell-type-specific density of MT1 and MT2 receptors in human pancreatic islets, which should be considered in context of the hormone secretion of islets, ii) the influence of diabetes on density of MT1 and MT2 as well as iii) the differential impact of melatonin on somatostatin secretion of nondiabetic and type 2 diabetic islets.

Melatonin receptors: molecular pharmacology and signalling in the context of system bias

Melatonin, N-acetyl-5-methoxytryptamine, an evolutionally old molecule, is produced by the pineal gland in vertebrates, and it binds with high affinity to melatonin receptors, which are members of the GPCR family. Among the multiple effects attributed to melatonin, we will focus here on those that are dependent on the activation of the two mammalian MT1 and MT2 melatonin receptors. We briefly summarize the latest developments on synthetic melatonin receptor ligands, including multi-target-directed ligands, and the characterization of signalling-biased ligands. We discuss signalling pathways activated by melatonin receptors that appear to be highly cell- and tissue-dependent, emphasizing the impact of system bias on the functional outcome. Different proteins have been demonstrated to interact with melatonin receptors, and thus, we postulate that part of this system bias has its molecular basis in differences of the expression of receptor-associated proteins including heterodimerization partners. Finally, bias at the level of the receptor, by the expression of genetic receptor variants, will be discussed to show how a modified receptor function can have an effect on the risk for common diseases like type 2 diabetes in humans. LINKED ARTICLES: This article is part of a themed section on Recent Developments in Research of Melatonin and its Potential Therapeutic Applications. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.16/issuetoc.

Circadian dysrhythmia-linked diabetes mellitus: Examining melatonin’s roles in prophylaxis and management

Diabetes mellitus is a chronic, life-threatening metabolic disorder that occurs worldwide. Despite an increase in the knowledge of the risk factors that are associated with diabetes mellitus, its worldwide prevalence has continued to rise; thus, necessitating more research into its aetiology. Recent researches are beginning to link a dysregulation of the circadian rhythm to impairment of intermediary metabolism; with evidences that circadian rhythm dysfunction might play an important role in the aetiology, course or prognosis of some cases of diabetes mellitus. These evidences thereby suggest possible relationships between the circadian rhythm regulator melatonin, and diabetes mellitus. In this review, we discuss the roles of the circadian rhythm in the regulation of the metabolism of carbohydrates and other macronutrients; with emphasis on the importance of melatonin and the impacts of its deficiency on carbohydrate homeostasis. Also, the possibility of using melatonin and its analogs for the “prophylaxis” or management of diabetes mellitus is also considered.

Mistimed food intake and sleep alters 24-hour time-of-day patterns of the human plasma proteome

Proteomics holds great promise for understanding human physiology, developing health biomarkers, and precision medicine. However, how much the plasma proteome varies with time of day and is regulated by the master circadian suprachiasmatic nucleus brain clock, assessed here by the melatonin rhythm, is largely unknown. Here, we assessed 24-h time-of-day patterns of human plasma proteins in six healthy men during daytime food intake and nighttime sleep in phase with the endogenous circadian clock (i.e., circadian alignment) versus daytime sleep and nighttime food intake out of phase with the endogenous circadian clock (i.e., circadian misalignment induced by simulated nightshift work). We identified 24-h time-of-day patterns in 573 of 1,129 proteins analyzed, with 30 proteins showing strong regulation by the circadian cycle. Relative to circadian alignment, the average abundance and/or 24-h time-of-day patterns of 127 proteins were altered during circadian misalignment. Altered proteins were associated with biological pathways involved in immune function, metabolism, and cancer. Of the 30 circadian-regulated proteins, the majority peaked between 1400 hours and 2100 hours, and these 30 proteins were associated with basic pathways involved in extracellular matrix organization, tyrosine kinase signaling, and signaling by receptor tyrosine-protein kinase erbB-2. Furthermore, circadian misalignment altered multiple proteins known to regulate glucose homeostasis and/or energy metabolism, with implications for altered metabolic physiology. Our findings demonstrate the circadian clock, the behavioral wake-sleep/food intake-fasting cycle, and interactions between these processes regulate 24-h time-of-day patterns of human plasma proteins and help identify mechanisms of circadian misalignment that may contribute to metabolic dysregulation.